Unlike a voice communication based service type, a high-speed multimedia data communication service is characterized in that data packets are not constantly arrived from a receiver but intermittently transmitted. Therefore, there may be a case where a lot of packets are suddenly arrived from the receiver. In order to provide a packet service to a plurality of users, resources such as time, symbol and power should commonly be used among the users.
Since a channel condition between a packet and its subsequent packet is varied, it is difficult to equally apply the existing power control to such a variable channel condition. Also, since the high-speed multimedia data communication service allows data, which are relatively more than those of the existing voice communication service type, to be transmitted and received during a transmission and reception interval, an advanced modulation scheme, various diversity schemes and error correction coding schemes are used. Among them, the existing link adaptation scheme for coping with channel condition variation between data packets will be described.
If channels of uplink/downlink are divided into a frequency domain, the channels are different from each other. Accordingly, a general link adaptation scheme is to apply a transmission and reception scheme most suitable for channel information by exchanging the channel information between a transmitter and a receiver. Representative examples of the existing link adaptation scheme include an adaptive modulation and coding (AMC) scheme and a power control scheme.
The power control scheme is to maintain transmission quality by controlling the power in accordance with a radio link, thereby ensuring quality of a link under a fixed transmission rate status such as the voice communication service. On the other hand, since a multimedia data service requires various transmission rates, various transmission qualities, and so on depending on service type, a link adaptation scheme which is different from that used in the existing voice based service is required. Since the AMC scheme is the link adaptation scheme efficient for multimedia data transmission, the AMC scheme is to vary a transmission rate (not transmission power) to be suitable for a channel environment. In case of the power control, the transmission power is varied depending on channel to obtain a fixed signal-to-Noise ratio (SNR) or signal-to-Interference ratio (SIR).
On the other hand, since AMC scheme determines a proper transmission rate depending on channel characteristics, the transmission power is basically fixed. The transmission rate is determined by MCS level, which relates to previously defined modulation and channel coding combination. The MCS level is determined depending on receiving SNR. The MCS level which satisfies a target frame error rate and has the highest efficiency is selected depending on the SNR. In order to support the AMC, a mobile station should know information of receiving SNR and forward channel quality information to a base station.
For example, a high speed downlink packet access (hereinafter, referred to as “HSDPA”) which is a wideband code division multiple access (WCDMA) wireless packet service uses a high speed dedicated physical control channel (hereinafter, referred to as “HS-DPCCH”), so that each user equipment can select a base station having the most excellent channel status to feed back modulation and coding information suitable for the corresponding channel, wherein the AMC scheme is applied to the WCDMA wireless packet service. A channel quality information (hereinafter, referred to as “CQI”) bit is allocated to the HS-DPCCH. The CQI bit is information fed back from a receiver to indicate the channel status. The CQI bit may represent the MCS level or may simply represent an appropriate SNR. Accordingly, the number of MCS levels, which is used by the transmitter and the receiver, may be varied depending on the information quantity of the CQI bit.
For example, in case of using MCS set as illustrated in Table 1 below, minimum 5 bits or greater are required for MCS set of all cases. In this case, the MCS set have a total of 32 MCS levels, and each combination of modulation and coding schemes from 0 to 31 is expressed by a decimal number, and is referred to as MCS index or CQI index.
For example, If CQI bit, which is fed back, is 01000, it represents MCS level having MCS index of 8 among the MCS set having a total of 32 MCS levels. This means that the base station transmits data to a mobile station by selecting a coding rate of ⅞ and a QPSK modulation scheme depending on the above feedback information. In other words, the system selects MCS level suitable for the channel status among the previously set MCS set, by using either channel information fed back from the receiver, or MCS index information (for example, CQI).
TABLE 1(CQI index)Coding RateModulation 0 (00000)1/5QPSK 1 (00001)1/4QPSK 2 (00010)1/3QPSK 3 (00011)1/2QPSK 4 (00100)3/5QPSK 5 (00101)2/3QPSK 6 (00110)3/4QPSK 7 (00111)4/5QPSK 8 (01000)7/8QPSK 9 (01001)1/216-QAM10 (01010)3/516-QAM11 (01011)2/316-QAM12 (01100)3/416-QAM13 (01101)4/516-QAM14 (01110)5/616-QAM15 (01111)7/816-QAM16 (10000)8/916-QAM17 (10001) 9/1016-QAM18 (10010)10/1116-QAM19 (10011)11/1216-QAM20 (10100)1/264-QAM21 (10101)3/564-QAM22 (10110)2/364-QAM23 (10111)3/464-QAM24 (11000)4/564-QAM25 (11001)5/664-QAM26 (11010)7/864-QAM27 (11011)8/964-QAM28 (11100) 9/1064-QAM29 (11101)10/1164-QAM30 (11110)11/1264-QAM31 (11111)164-QAM
Generally, CQI information for sharing channel information between the base station and the mobile station is mainly used for a frequency division duplexing (hereinafter, referred to as “FDD”) system which has different channels between an uplink and a downlink. The CQI which is fed back is used for determination of the MCS level and scheduling of the base station. The channel information or the MCS level index information may be fed back at a constant time period, or may be fed back when there is a request of the mobile station. The channel information or the MCS index information is transmitted using an error correction code (hereinafter, referred to as “ECC”) to enhance reliability. The transmitter notifies the receiver of the selected MCS level through the downlink by considering feedback of the receiver.
An orthogonal frequency division multiplexing (hereinafter, referred to as “OFDM”) system can have different channels depending on a frequency band, and can apply different MCSs depending on the frequency band. Therefore, the OFDM system uses a link adaptation scheme by feeding back channel information at a constant frequency band unit.
Also, since a system which uses a multiple input multiple output (MIMO) scheme can apply different MCS levels to each signal which is spatially transmitted, the system may feed back channel information spatially. The existing AMC scheme uses adaptive MCS level depending on the channel status. Namely, the existing AMC scheme uses the adaptive MCS level by fixing one MCS set between the system and the mobile station. Accordingly, channel information of the same quantity is always fed back, and the transmitter notifies the selected MCS level through the downlink.
Therefore, in case that a channel is slowly varied due to slow moving speed of the mobile station and the MCS level which sufficiently considers the channel status through the AMC can be used, MCS set having small MCS granularity can use a proper MCS level per receiving SNR, and higher system performance can be obtained. Also, a closed-loop multiple antenna scheme such as beam-forming and preceding, which is suitable for the higher system performance, can be used.
However, in case that a channel is fast varied due to fast moving speed of the mobile station, or in an open-loop system such as diversity scheme which does not require feedback information except for CQI information, since it is likely that channel information measured during reception is greatly varied, if MCS granularity is small, the system may fail to transmit data at a required transmission rate within a fast time. In this respect, the MCS granularity should be greater than 1 dB interval.
Furthermore, the high speed wireless data service requires fast data processing in view of its characteristics as described above. For example, in high speed data packet access (HSDPA) of the WCDMA, a part which manages and controls the AMC and hybrid automatic repeat request (hereinafter, referred to as “HARQ”) should be located close to a wireless interface, thereby efficiently operating the AMC and the HARQ. In the existing voice based communication, since a part which is in charge of scheduling of data is located in a radio network controller (RNC), latency in processing time occurs.
For this reason, methods for properly coping with variation of the channel environment and enhancing efficiency are required. Also, in order to reduce complexity of the system due to high speed data, it is necessary to reduce feedback overhead of the mobile station. This is likewise applied to multicast traffic.